Various hybrid powertrain systems use electrical energy storage devices to supply electrical energy to electrical machines, which are operable to provide motive torque, often in conjunction with an internal combustion engine. One such hybrid powertrain architecture comprises a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving power from a prime mover power source and an output member for delivering power from the transmission to a vehicle driveline. First and second electric machines, i.e. motor/generators, are operatively connected to an energy storage device for interchanging electrical power therebetween. A control unit is provided for regulating the electrical power interchange between the energy storage device and the electric machines. The control unit also regulates electrical power interchange between the first and second electric machines.
One of the design considerations in vehicle powertrain systems is an ability to provide consistent vehicle performance and component/system service life. Hybrid vehicles, and more specifically the battery pack systems utilized therewith, provide vehicle system designers with new challenges and tradeoffs. It has been observed that service life of an electrical energy storage device, e.g. a battery pack system, increases as resting temperature of the battery pack decreases. However, cold operating temperature introduces limits in battery charge/discharge performance until temperature of the pack is increased. A warm battery pack is more able to supply required power to the vehicle propulsion system, but continued warm temperature operation may result in diminished service life.
Modern hybrid vehicle systems manage various aspects of operation of the hybrid system to effect improved service life of the battery. For example, depth of battery discharge is managed, amp-hour (A-h) throughput is limited, and convection fans are used to cool the battery pack. Ambient environmental conditions in which the vehicle is operated has largely been ignored. However, the ambient environmental conditions may have significant effect upon battery service life. Specifically, same models of hybrid vehicles released into various geographic areas throughout North America would likely not result in the same battery pack life, even if all the vehicles were driven on the same cycle. The vehicle's environment must be considered if a useful estimation of battery life is to be derived. Additionally, customer expectations, competition and government regulations impose standards of performance, including for service life of battery packs, which must be met.
End of service life of a battery pack may be indicated by ohmic resistance of the battery pack. The ohmic resistance of the battery pack is typically flat during much of the service life of the vehicle and battery pack however, thus preventing a reliable estimate of real-time state-of-life (‘SOL’) of the battery pack throughout most of the service life. Instead, ohmic resistance is most useful to indicate incipient end of service life of the battery pack.
Furthermore, service life of a battery pack is affected by resting temperature, i.e. life of a battery pack system increases as resting temperature of the battery pack decreases. Therefore a battery pack control system that is operable to determine a state-of-life of a monitored battery pack would benefit from a parametric value that is indicative of an effect of temperature of the battery pack during quiescent or stasis periods. Such quiescent periods occur when the battery pack is neither charging nor discharging, e.g. when a hybrid vehicle using the battery pack is shutdown.
Therefore, it would be useful to have a method and apparatus which determines an effect of temperature during a quiescent period on life expectancy of a battery pack for a hybrid vehicle.